Recently, interests in energy storage technologies have been increasingly grown, and, while the application of the energy storage technologies is expanded to mobile phones, camcorders, notebook PCs, and even to electric vehicles, efforts for research and development of electrochemical devices have been gradually materialized.
The electrochemical devices have received most attention in this respect, and there emerges an interest in rechargeable secondary batteries among these electrochemical devices. Particularly, among the currently used secondary batteries, lithium secondary batteries, developed in the early 1990's, are spotlighted because the lithium secondary batteries may have higher operating voltage and significantly higher energy density.
The lithium secondary batteries are composed of a negative electrode formed of such as a carbon material capable of intercalating and deintercalating lithium ions, a positive electrode formed of such as a lithium-containing oxide, and a non-aqueous electrolyte solution.
The lithium secondary batteries may be categorized into a lithium ion liquid battery (LiLB) using a liquid electrolyte, a lithium ion polymer battery (LiPB) using a gel-type polymer electrolyte, and a lithium polymer battery (LPB) using a solid polymer electrolyte according to a type of the electrolyte used.
Recently, as the application range of the lithium secondary batteries has expanded, there is a growing demand for lithium secondary batteries which may be safely charged even at a high voltage while maintaining excellent cycle life characteristics even in more harsh environments such as a high temperature or low temperature environment and high-voltage charging.
However, the lithium secondary battery using a liquid electrolyte may have limitations in that a structure of the battery may be deformed due to the generation of gas caused by the oxidation of the electrolyte during long-term storage at high temperatures, or fire and explosion of the battery may occur due to internal heating caused by overcharging. For example, a positive electrode structure collapses while the voltage increases during the overcharging, and accordingly, the negative electrode is degraded while metal ions dissolved from the surface of the positive electrode are electrodeposited on the negative electrode. Such a battery performance degradation phenomenon tends to be more accelerated when the potential of the positive electrode is increased or the battery is exposed at high temperatures.
In order to address the above-described limitations, a method has been proposed in which a material, which protects the positive electrode by forming a film on the positive electrode, is added to the electrolyte solution.